Experimental Design Modelling and Optimization of Triazine Herbicides Removal With Reduced Graphene Oxide Using Response Surface Method

In this work, triazines were chosen as the organic micropollutants model, to develop a useful method for the removal of 85 triazine products, using a reduced derivative of graphene oxide as adsorbent material. The pristine graphene oxide and its 86 thermally reduced derivatives under mild conditions were tested, optimizing the GO reduction conditions by means of 87 DOE coupled with the response surface methodology. For the reduction it was decided to choose the mildest and simplest 88 conditions possible, using an air heat treatment in a common laboratory oven. The optimal reduction conditions deduced 89 from the response surface were calculated at a reduction temperature of 110 °C maintained for 24 hours and rGO sample 90 was employed in the adsorption of the triazines. All the adsorbent materials have been characterized before use, by 91 Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), X- 92 ray photoelectron spectroscopy (XPS) and Brunauer-Emmett- Teller (BET) surface area analysis. Triazine analyses were 93 performed by HPLC. The data obtained from the adsorption isotherms have been fitted with the Langmuir and Freundlich 94 models, and the Freundlich model was the best one, especially for the Atraton and the Prometryn. The maximum 95 adsorption capacity obtained was 4.4 mg/g for Atrazine, 19.4 mg/g for Atraton and 18.4 mg/g for Prometryn, at room 96 temperature.

The adsorption of organic pollutants is one of the promising methodologies for their removal from the environmental, 112 and the interest towards efficient and low-cost materials for remediation of contaminants from water is strongly emerging 113 (

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One of the main causes of pollution of surface and groundwater is attributed to the increase in the use of herbicides in 119 agricultural activities, causing great concern due to the potential risk to human health (Jablonowski, Schäffer, and Burauel 120 2011; Sousa et al. 2018).

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Atrazine is an herbicide, of the triazine classes, typically used for the control of broadleaf season-long weeds in a variety 122 of crops such as corn and sugarcane, but it also finds use on turfs such as golf courses and residential lawns as well (Frank 123 and Sirons 1979; Miller et al. 2000). Human exposure to atrazine is linked to several serious health effects. A potent 124 endocrine disrupter, atrazine interferes with hormonal activity of animals and humans at extremely low doses (Sanderson 125 et al. 2002). It exhibits acute, chronic and phytotoxicity. It has been proved that atrazine contains mutagenic and

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In this work, DOE coupled with RSM was chosen to optimize the graphene oxide reduction conditions in order to develop 165 a useful method for the removal of triazine pesticides from the aqueous medium. The rGO material was synthesized from 166 graphene oxide produced in the laboratory, after a mild heat treatment of the pristine GO.

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Atrazine was chosen as the organic micropollutant model for the standardised batch adsorption tests and a three-level full 168 factorial design was employed to plan the representative experiments. Eventually, the optimised sorbent material was 169 tested on other triazine models namely Atraton and Prometryn. The idea behind the work was to couple an optimization 170 step to the study of the triazine adsorption onto rGO; this was achieved by testing, according to the three-level full factorial 171 design, graphene oxide derivatives thermally reduced. The objective was to use a simple and environmental friendly 172 sorbent material that was optimised with a minimum number of experiments and in mild conditions, which are easily 173 controllable and do not require an inert atmosphere. As far as our knowledge there are no studies that involve simple rGO 174 films for atrazine adsorption.

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Many groups used magnetic graphene oxide-based nanocomposites for the removal of pollutants for sustainable water 176 purification. Zhao et al reported pioneering work on the use of a graphene-based Fe3O4 magnetic nanoparticles as the 177 adsorbent for the magnetic solid-phase extraction of some triazine herbicides in environmental water (Zhao et al. 2011) 178 followed by high performance liquid chromatography. 179 Boruah et al. used Fe3O4/reduced graphene oxide (rGO) nano composite, which is easily and quickly separated from the 180 aqueous medium using an external magnet for its reuse (Boruah et al. 2017

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(5 g), and sodium nitrate (3.8 g) were placed into a beaker in a salt/ice bath. Subsequently, 375 ml of concentrated sulfuric 212 acid was added. The reaction mixture was kept in continuous agitation by a mechanical stirrer. After the mixture has 213 become homogeneous, 25 g of potassium permanganate were slowly added. The solution was kept under stirring for 5 214 days at room temperature. After 5 days a 5% H 2 SO 4 aqueous solution (700 ml) was poured through a funnel and H 2 O 2 215 (30 wt%) was added drop by drop to remove the potassium permanganate and the suspension was thus stirred for another 216 2 hr. In order to obtain a clean product, the mixture was diluted with 5% H 2 SO 4 (2 l) and left to settle for 1 day. Inorganic 217 impurities were removed through successive centrifuges, after removing the surnatant. The solid part was 218 washed/centrifuged at 4000 rpm for 10 min with a 5% aqueous solution of H 2 SO 4 and H 2 O 2 at 0.3% (12 times), then 4% 219 HCl (3 times), deionized water (8 times), and finally MilliQ water (2 times), removing the supernatant after each passage.

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The pH of the dispersion was monitored until it reached 6-7. Finally, the GO is transferred to acetone and dried at 50 °C     The elution was performed at room temperature, constant flowrate (1 ml/min) and isocratic conditions using a mixture 251 (35:65, v/v) of water and acetonitrile. The chromatographic apparatus was controlled by Empower software (Waters).

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The analysed solutions were filtered by HPLC filters Whatman Spartan13/02 RC.

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The batch triazines adsorption experiments were carried out at room temperature under shaker conditions. Ten milligrams 255 of rGO film were placed in contact with 10 ml of ultrapure water, in screw-cap glass vials, containing a single triazine.

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Sorption isotherm experiments were conducted with seven initial pesticide concentrations (0.5, 1.0, 2.0, 5.0, 10, 20, and 257 50 μg/ml). The point at the concentration of 10 μg/ml was repeated in triplicate. The vials of the nine samples containing 258 different concentrations of pesticide were simultaneously placed on orbital shaker at 300 rpm in the dark for 1h. After 259 reaching equilibrium, 1 ml of solution was collected, filtered with 0.2 μm PTFE filters (PHENEX, Phenomenex) and 260 placed in HPLC vials to determine the equilibrium concentration (C eq ). Preliminary kinetic tests were achieved, and 261 equilibrium was assumed when no further change in pesticide up-take was observed. Based on these experiments 1 h 262 contact time was sufficient to reach equilibrium.

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The adsorptions data can be understood using several approaches. The models usually applied are the Freundlich and

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The presence of a well oxidized starting material can influence the subsequent thermal reduction that has been chosen. In

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In this work, the adsorption of atrazine was carried out starting from the pristine GO material and on some of its thermally 354 reduced derivatives at a temperature between 80 and 120 °C, and considering a reduction time varying between 18 and 355 30 hours total.

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To achieve the maximum adsorption capacity, the best reduction conditions were determined to obtain an optimal sorbent.

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An experimental multivariate design with two independent variables, time (t) and temperature (T), was used.    Table 02). In the 110 °C reduced rGO we found an increasing in the

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However, the presence of a crumpled three-dimensional structure of the sheets still leaves many exposed surface areas.

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The adsorption parameters obtained by applying both models to each of the examined herbicide and the determination 509 coefficients (R 2 ) of the linear fits are summarized in adsorption of the Atraton. The agreement between the experimental data and those of the model is also confirmed by the small uncertainties calculated on the parameters 1/n and K F (Table 03). These findings thus effectively demonstrate the 518 heterogeneous enrichment of the triazines on the rGO edges and a multilayer adsorption on the surface of rGO. In the 519 Freundlich models the values of the parameter n are more than 1, this fact indicates that the adsorption process is 520 favourable. The observed trends are characterised by an increase in the quantity adsorbed as concentration rise. The 521 tendency to reach saturation can be understood as a measure of the maximum adsorbing capacity of the material.

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Furthermore, the values of q max reported in Table 03 show that Atraton is adsorbed more than Prometryn and Atrazine.

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Usually, the adsorption of analogous compounds follows the trend predicted by the Lundelius rule, which establishes a 535 general criterion in which a compound is less adsorbable the higher its solubility in the solvent. This can be explained by 536 considering that the higher the solubility, the stronger the solute-solvent bond and, therefore, the lower the adsorption 537 capacity. In our case, however, we find an inverse order of adsorption of the triazines, since Atraton is more adsorbed 538 than Prometryn while Atrazine is the least adsorbed. The solubility of atrazine is the lowest among the compounds studied, 539 and is equal to 33 ppm at 27 °C. In our case we attribute this behaviour to the fact that the adsorbent material still contains 540 a high number of oxygenated sites on its skeleton that can form hydrogen bonds with the analytes. But analysing the 541 structure of the three triazines, it is evident that the triazine ring is common to the three analytes, and forms hydrogen The adsorption yield is not the highest possible, but the graphenic material is a versatile platform, and can also provide 578 for subsequent chemical functionalization, by means of well-known synthetic strategies.

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This preliminary work can be used to further optimize graphenic materials, choosing which conditions may be the best 580 for the adsorption of different analytes, and preparing the respective GO derivatives that best respond to the adsorption 581 characteristics of the pollutants. By introducing functional groups that modify the surface charge of the material itself we 582 expected an improvement in adsorption capacity of sorbent material.

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Acknowledgments We would like to thank all participants who contributed their time and completed laboratory study.    Adsorption isotherm plots described according to the linearized adsorption models of Langmuir (a) and Freundlich (b) and reported for all the involved triazine.

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